Embed a PNG in HTML by placing its Base64 bytes in a data: URL:
<img src="data:image/png;base64,ENCODED_PNG_BYTES" alt="Description of the image">The image/png media type identifies the file, ;base64 says the payload is encoded, and the comma separates that declaration from the encoded bytes. For a file chosen by a user, the safest browser-native conversion is FileReader.readAsDataURL().
The data URL format
RFC 2397 defines the inline URL grammar as data:[<media-type>][;base64],<data>. A PNG image therefore uses:
data:image/png;base64,ENCODED_PNG_BYTESUse that entire value as the src of an image. The comma is required; without it, the browser cannot reliably distinguish the metadata from the payload.
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| Part | Purpose | PNG example |
|---|---|---|
data: |
Identifies a data URL. | data: |
| Media type | Tells the browser what the bytes represent. | image/png |
;base64 |
Declares that the following data is Base64 text. | ;base64 |
| Comma | Separates the declaration from the encoded payload. | , |
| Payload | The PNG file’s encoded bytes. | iVBORw0KGgo... |
Keep meaningful alternative text in alt. Base64 does not make an image accessible by itself; the surrounding HTML still needs an appropriate text alternative.
Convert a selected PNG in the browser
FileReader.readAsDataURL() accepts a Blob or File and, when reading finishes, places a complete data URL in reader.result. This single HTML file lets someone choose a PNG and immediately preview it:
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>PNG Base64 preview</title>
</head>
<body>
<label for="pngFile">Choose a PNG</label>
<input id="pngFile" type="file" accept="image/png">
<img id="preview" alt="Selected PNG preview">
<script>
const input = document.querySelector('#pngFile');
const preview = document.querySelector('#preview');
input.addEventListener('change', () => {
const file = input.files[0];
if (!file) return;
const reader = new FileReader();
reader.addEventListener('load', () => {
preview.src = reader.result;
});
reader.addEventListener('error', () => {
console.error('Could not read the selected file.');
});
reader.readAsDataURL(file);
});
</script>
</body>
</html>What happens in this example
- The
accept="image/png"hint filters the file picker toward PNG files. It is a user-interface hint, not a security boundary, so validate the file again if the upload will be sent to a server. - The
changeevent runs after a selection. If the user cancels,input.files[0]is absent and the handler returns. readAsDataURL(file)reads the binary file asynchronously.- The
loadevent exposes the complete string, includingdata:image/png;base64,. Assigning it directly topreview.srcdisplays the image.
Get only the raw Base64 payload
Most HTML needs the complete data URL. Remove the prefix only when another API explicitly asks for raw Base64:
const dataUrl = reader.result;
const comma = dataUrl.indexOf(',');
const base64Only = comma === -1 ? '' : dataUrl.slice(comma + 1);MDN documents the equivalent pattern of removing data:*/*;base64, through the comma. Do not encode reader.result again: it is already a data URL and a second Base64 pass produces the wrong bytes.
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A reusable Promise helper
For application code, wrapping the reader in a Promise makes it easy to await the conversion:
function pngFileToDataUrl(file) {
return new Promise((resolve, reject) => {
const reader = new FileReader();
reader.addEventListener('load', () => resolve(reader.result));
reader.addEventListener('error', () => reject(reader.error));
reader.readAsDataURL(file);
});
}
input.addEventListener('change', async () => {
const file = input.files[0];
if (!file) return;
try {
preview.src = await pngFileToDataUrl(file);
} catch (error) {
console.error(error);
}
});Why not just use btoa()?
btoa() and atob() operate on binary strings. They are useful when you already have bytes represented as a suitable binary string, but btoa() is not a general Unicode encoder: characters outside its byte range can throw an exception. A PNG is binary data, and a File is asynchronous, so readAsDataURL() avoids converting the file through an unsafe text representation.
If you deliberately use lower-level byte handling, first obtain bytes from an ArrayBuffer and convert those bytes to a binary string or use a byte-aware Base64 implementation. Do not pass arbitrary human-readable Unicode text to btoa() and assume the result represents a PNG.
Debug a broken Base64 PNG
When an image is missing or appears as a broken-image icon, inspect the value assigned to src and check these failure modes:
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The prefix is wrong
For a PNG, the beginning must be exactly data:image/png;base64,. A misspelled media type, an omitted ;base64 marker, or malformed media parameters can be ignored or rejected without a useful console message.
The comma is missing
The comma terminates the metadata. Keep it between the prefix and the encoded bytes.
The value was encoded twice
A string returned by readAsDataURL() already contains both the prefix and Base64 payload. Put it directly in src, or remove only the prefix when raw Base64 is required. Never call btoa() on the complete data URL.
The payload was truncated or altered
Copy the entire encoded string. Truncation, accidental edits, or inserted whitespace can corrupt the decoded PNG. Store the value in a variable or generate it at runtime rather than manually retyping a long literal.
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The selected object is empty
Check input.files[0] before reading. A cancel action, a reset input, or a failed file selection leaves it empty.
The file is not actually a PNG
The accept attribute does not validate file contents. If correctness matters, inspect the file on the server or decode it before accepting it. A renamed non-PNG file can still be selected by a user.
Inline Base64 versus a normal image URL
Use a data URL when the document should be self-contained: small prototypes, single-file demos, generated HTML, or an artifact that must travel as one document. Use a normal URL when the image is reused, updated independently, or large enough that keeping it outside the HTML improves delivery and caching.
| Consideration | Base64 data URL | Normal src URL |
|---|---|---|
| Deployment | One document contains markup and image bytes. | HTML and the image are separate assets. |
| Caching | The embedded bytes are tied to that document. | The image can be cached and reused independently. |
| Updates | Changing the image requires changing the document. | Replace the asset without rewriting every HTML document. |
| Readability | Source contains a long opaque string. | Source remains a short, recognizable path or URL. |
| Size | The HTML becomes larger because it carries encoded bytes. | The binary remains a separate response. |
| Origin and security | Modern browsers treat data URLs as unique opaque origins; top-level navigation to data: is blocked in modern browsers. |
The resource follows the normal origin and request rules of its URL. |
There is no universal browser byte limit for data URLs that you can safely assume across implementations. If a document approaches a browser’s practical limits, switch to a normal asset URL and verify behavior in the browsers you support.
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Inspect and test the result
- Open developer tools and inspect the image element.
- Confirm that
srcstarts withdata:image/png;base64,. - Use the console to check
preview.completeandpreview.naturalWidth. A decoded image normally has a positive natural width. - If the value came from a server, verify that the response contains the PNG bytes you expect before encoding them.
- Keep
alttext meaningful even when the image is only decorative; use an emptyalt=""when it truly conveys no information.
Or skip the browser setup
If what you need first is a clean website screenshot to place into an HTML document, ScreenshotNeo returns a screenshot with one GET request. Before capture it accepts cookie or consent banners as a visitor and removes more than 60 known consent platforms, newsletter popups, and chat widgets; each cleanup step can be disabled. Only clean shots are billed, while bot checks or CAPTCHAs, blank pages, timeouts, failed loads, and cache hits cost nothing. The response identifies the result with X-Page-Verdict and X-Billed headers. Its MCP server also lets Claude, Cursor, or another MCP client call take_screenshot, get_page_info, and capture_pdf.
See the ScreenshotNeo API documentation for request options. The following examples use the supplied API endpoint and save the returned asset locally.
cURL
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)Node.js
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);ScreenshotNeo has a free plan with 1,000 screenshots each month and no card required. Paid plans start at $5 for 3,000 shots; yearly billing gives two months free, and every feature is available on every plan. After you receive an image, convert that local file with the browser workflow above or your normal server-side Base64 encoder, then place the resulting data URL in src.
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Frequently Asked Questions
Can I use a Base64 PNG in an email?
A self-contained data URL can be useful for single-file artifacts such as some HTML email workflows, but email clients apply their own filtering and size policies. Test the exact clients and message pipeline you support.
Should I store the Base64 string in my database?
Only do so when the application specifically benefits from a self-contained representation. For reusable or frequently updated images, storing the binary as an asset and serving a normal URL keeps HTML and records smaller and independently cacheable.
How do I turn the data URL back into a PNG file?
Split at the first comma, decode the part after it as Base64, and write the resulting bytes to a file with a .png extension. Preserve the image/png declaration so your code can reject an unexpected media type.
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